1. Field of the Invention
The present invention relates to mobile stations which operate in wireless communication systems and, more particularly, to certain improvements allowing for more rapid settling and/or better phase noise performance of frequency synthesizers used in such mobile stations.
2. Related Prior Art Systems
The prior art includes cellular radio systems which have been operating in the United States since the early 1980s, and providing telephone service to an ever growing subscriber base, presently estimated at over 20 million subscribers. Cellular telephone service operates much like the fixed, wireline telephone service in homes and offices, except that radio frequencies rather than telephone wires are used to connect telephone calls to and from the mobile subscribers. Each mobile subscriber is assigned a private (10 digit) directory telephone number and is usually billed based on the amount of "airtime" he or she spends talking on the cellular telephone each month. Many of the service features available to landline telephone users (e.g., call waiting, call forwarding, three-way calling, etc.) are also generally available to mobile subscribers.
In the United States, cellular operating licenses have been awarded by the Federal Communications Commission (FCC) pursuant to a licensing scheme which divided the country into geographic service markets defined according to the 1980 Census. The major metropolitan markets are called metropolitan statistical areas (MSAs) while the smaller rural markets are called rural statistical areas (RSAs). Only two cellular licenses are awarded for operating systems in each market. These two systems, which are commonly referred to as the "A-system" and the "B-system," respectively, are assigned different radio frequency (RF) bands (blocks) in the 800 MHz range. Mobile subscribers have the freedom to subscribe to service from either the A-system or the B-system operator (or both). The local system from which service is subscribed is called the "home" system. When travelling outside the home system, a mobile subscriber may be able to obtain service in a distant system if there is a "roaming" agreement between the operators of the home and "visited" systems.
The architecture for a typical cellular radio system is shown in FIG. 1. A geographical area (e.g., a metropolitan area) is divided into several smaller, contiguous radio coverage areas, called "cells," such as cells C1-C10. The cells C1-C10 are served by a corresponding group of fixed radio stations, called "base stations," B1-B10, each of which includes a plurality of RF channel units (transceivers) that operate on a subset of the RF channels assigned to the system, as well known in the art. For illustration purposes, the base stations B1-B10 are shown in FIG. 1 to be located at the center of the cells C1-C10, respectively, and are shown to be equipped with omni-directional antennas transmitting equally in all directions. However, the base stations B1-B10 may also be located near the periphery or otherwise away from the centers of the cells C1-C10, and may illuminate the cells C1-C10 with radio signals directionally (e.g., a base station may be equipped with three directional antennas each covering a 120 degrees sector).
The RF channels allocated to any given cell (or sector) may be reallocated to a distant cell in accordance with a frequency reuse plan as is well known in the art. In each cell (or sector), at least one RF channel is used to carry control or supervisory messages, and is called the "control" or "paging/access" channel. The other RF channels are used to carry voice conversations, and are called the "voice" or "speech" channels. The cellular telephone users (mobile subscribers) in the cells C1-C10 are provided with portable (hand-held), transportable (hand-carried) or mobile (car-mounted) telephone units, collectively referred to as "mobile stations," such as mobile stations M1-M5, each of which communicates with a nearby base station. Each of the mobile stations M1-M5 includes a controller (microprocessor) and a transceiver, as well known in the art. The transceiver in each mobile station may tune to any of the RF channels specified in the system (whereas each of the transceivers in the base stations B1-B10 usually operates on only one of the different RF channels used in the corresponding cell).
With continuing reference to FIG. 1, the base stations B1-B10 are connected to and controlled by a mobile telephone switching office (MTSO) 20. The MTSO 20, in turn, is connected to a central office (not specifically shown in FIG. 1) in the landline (wireline) public switched telephone network (PSTN) 22, or to a similar facility such as an integrated system digital network (ISDN). The MTSO 20 switches calls between wireline and mobile subscribers, controls signalling to the mobile stations M1-M5, compiles billing statistics, stores subscriber service profiles, and provides for the operation, maintenance and testing of the system.
Access to the cellular system of FIG. 1 by any of the mobile stations M1-M5 is controlled on the basis of a mobile identification number (MIN) and an electronic serial number (ESN) which are stored in the mobile station. The MIN is a digital representation of the 10-digit directory telephone number assigned to each mobile subscriber by the home system operator. The electronic serial number (ESN) is assigned by the manufacturer and permanently stored in the mobile station. The MIN/ESN pair is sent from the mobile station when originating a call and its validity is checked by the MTSO 20. If the MIN/ESN pair is determined to be invalid (e.g., if the ESN has been blacklisted because the mobile station was reported to be stolen), the system may deny access to the mobile station.
When turned on (powered up), each of the mobile stations M1-M5 enters the idle state (standby mode) and tunes to and continuously monitors the strongest control channel (generally, the control channel of the cell in which the mobile station is located at that moment). When moving between cells while in the idle state, the mobile station will eventually "lose" radio connection on the control channel of the "old" cell and tune to the control channel of the "new" cell. The initial tuning to, and the change of, control channel are both accomplished automatically by scanning all the control channels in operation in the cellular system to find the "best" control channel (in the United States, there are 21 "dedicated" control channels in each cellular system which means that the mobile station has to scan a maximum number of 21 RF channels). When a control channel with good reception quality is found, the mobile station remains tuned to this channel until the quality deteriorates again. In this manner, the mobile station remains "in touch" with the system and may receive or initiate a telephone call through one of the base stations B1-B10 which is connected to the MTSO 20.
To detect incoming calls, the mobile station continuously monitors the current control channel to determine whether a page message addressed to it (i.e., containing its MIN) has been received. A page message will be sent to the mobile station, for example, when an ordinary (landline) subscriber calls the mobile subscriber. The call is directed from the PSTN 22 to the MTSO 20 where the dialed number is analyzed. If the dialed number is validated, the MTSO 20 requests some or all of the base stations B1-B10 to page the called mobile station throughout their corresponding cells C1-C10. Each of the base stations B1-B10 which receive the request from the MTSO 20 will then transmit over the control channel of the corresponding cell a page message containing the MIN of the called mobile station. Each of the idle mobile stations M1-M5 which is present in that cell will compare the MIN in the page message received over the control channel with the MIN stored in the mobile station. The called mobile station with the matching MIN will automatically transmit a page response over the control channel to the base station, which then forwards the page response to the MTSO 20. Upon receiving the page response, the MTSO 20 selects an available voice channel in the cell from which the page response was received (the MTSO 20 maintains an idle channel list for this purpose), and requests the base station in that cell to order the mobile station via the control channel to tune to the selected voice channel. A through-connection is established once the mobile station has tuned to the selected voice channel.
When, on the other hand, a mobile subscriber initiates a call (e.g., by dialing the telephone number of an ordinary subscriber and pressing the "send" button on the telephone handset in the mobile station), the dialed number and MIN/ESN pair for the mobile station are sent over the control channel to the base station and forwarded to the MTSO 20, which validates the mobile station, assigns a voice channel and establishes a through-connection for the conversation as described before. If the mobile station moves between cells while in the conversation state, the MTSO 20 will perform a "handoff" of the call from the old base station to the new base station. The MTSO 20 selects an available voice channel in the new cell and then orders the old base station to send to the mobile station on the current voice channel in the old cell a handoff message which informs the mobile station to tune to the selected voice channel in the new cell. The handoff message is sent in a "blank and burst" mode which causes a short but hardly noticeable break in the conversation. Upon receipt of the handoff message, the mobile station tunes to the new voice channel and a through-connection is established by the MTSO 20 via the new cell. The old voice channel in the old cell is marked idle in the MTSO 20 and may be used for another conversation. Furthermore, when travelling outside the system, the mobile station may be handed off to a cell in an adjacent system if there is a roaming agreement between the operators of the two systems.
The original cellular radio systems, as described generally above, used analog transmission methods, specifically frequency modulation (FM), and duplex (two-way) RF channels in accordance with the Advanced Mobile Phone Service (AMPS) standard. According to the AMPS standard, each control or voice channel between the base station and the mobile station uses a pair of separate frequencies consisting of a forward (downlink) frequency for transmission by the base station (reception by the mobile station) and a reverse (uplink) frequency for transmission by the mobile station (reception by the base station). The AMPS system, therefore, is a single-channel-per-carrier (SCPC) system allowing for only one voice circuit (telephone conversation) per RF channel. Different users are provided access to the same set of RF channels with each user being assigned a different RF channel (pair of frequencies) in a technique known as frequency division multiple access (FDMA). This original AMPS (analog) architecture forms the basis for an industry standard sponsored by the Electronics Industries Association (EIA) and the Telecommunication Industry Association (TIA), and known as EIA/TIA-553.
In the late 1980s, however, the cellular industry in the United States began migrating from analog to digital technology, motivated in large part by the need to address the steady growth in the subscriber population and the increasing demand on system capacity. It was recognized early on that the capacity improvements sought for the next generation cellular systems could be achieved by either "cell splitting" to provide more channels per subscribers in the specific areas where increased capacity is needed, or by the use of more advanced digital radio technology in those areas, or by a combination of both approaches. According to the first approach (cell splitting), by reducing the transmit power of the base station, the size of the corresponding cell (or cell radius) and, with it, the frequency reuse distance are reduced thereby resulting in more channels per geographic area (i.e., increased capacity). Additional benefits of a smaller cell include a longer "talk time" for the user since the mobile station will use substantially lower transmit power than in a larger cell and, consequently, its battery will not need to be recharged as often.
While cell splitting held the promise of improving both capacity and coverage for the growing mobile subscriber base, the actual capacity gains were limited by the use of the analog AMPS technology. It was commonly believed that the desired capacity gains, and indeed the effectiveness of the microcellular (cell splitting) concept in increasing capacity, can be maximized only by the use of digital technology. Thus, in an effort to go digital, the EIA/TIA developed a number of air interface standards which use digital voice encoding (analog-to-digital conversion and voice compression) and time division multiple access (TDMA) or code division multiple access (CDMA) techniques to multiply the number of voice circuits (conversations) per RF channel (i.e., to increase capacity). These standards include IS-54 (TDMA) and IS-95 (CDMA), both of which are "dual mode" standards in that they support the use of the original AMPS analog voice and control channels in addition to digital speech channels defined within the existing AMPS framework (so as to ease the transition from analog to digital and to allow the continued use of existing analog mobile stations).
The dual-mode IS-54 standard, in particular, has become known as the digital AMPS (D-AMPS) standard. More recently, the EIA/TIA has developed a new specification for D-AMPS, which includes a digital control channel suitable for supporting public or private microcell operation, extended mobile station battery life, and enhanced end-user features. This new specification builds on the IS-54B standard (the current revision of IS-54), and it is known as IS-136. (All of the foregoing EIA/TIA standards are hereby incorporated herein by reference as may be necessary for a full understanding of these background developments. Copies of these standards may be obtained from the Electronics Industries Association, 2001 Pennsylvania Avenue, N.W., Washington, D.C. 20006).
According to IS-54B and as shown in FIG. 2, each RF channel is time division multiplexed (TDM) into a series of repeating time slots which are grouped into frames carrying from three to six digital speech channels (three to six telephone conversations) depending on the source rate of the speech coder used for each digital speech channel. Each frame on the RF channel comprises six equally sized time slots (1-6) and is 40 ms long (i.e, there are 25 frames per second). The speech coder for each digital traffic channel (DTCH) can operate at either full-rate or half-rate. A full-rate DTCH uses two equally spaced slots of the frame (i.e., slots 1 and 4, or slots 2 and 5, or slots 3 and 6). When operating at full-rate, the RF channel may be assigned to three users (A-C). Thus, for example, user A is assigned to slots 1 and 4, user B is assigned to slots 2 and 5, and user C is assigned to slots 3 and 6 of the frame as shown in FIG. 2. Each half-rate DTCH uses only one time slot of the frame. At half-rate, the RF channel may be assigned to six users (A-F) with each user being assigned to one of the six slots of the frame as also shown in FIG. 2. Thus, it can be seen that the DTCH as specified in the IS-54B standard allows for an increase in capacity of from three to six times that of the analog RF channel. At call set-up or handoff, a dual-mode mobile station will be assigned preferably to a digital traffic channel (DTCH) and, if none is available, it can be assigned to an analog voice channel (AVC). An analog-only mobile station, however, can only be assigned to an AVC.
The IS-136 standard specifies a digital control channel (DCCH) which is defined similarly to the digital traffic channel (DTCH) specified in IS-54B (i.e., on the same set of RF channels and with the same TDMA frame format and slot size). Referring back to FIG. 2, a half-rate DCCH would occupy one slot while a full-rate DCCH would occupy two slots out of the six slots in each 40 ms frame. The DCCH slots may then be mapped into different logical channels which are organized into a series of superframes. FIG. 3 shows the superframe structure of a full-rate DCCH according to IS-136 (in this example, the DCCH is defined over channel "A" in the TDMA frame). A superframe is defined in IS-136 as the collection of 32 consecutive time slots (640 ms) for a full-rate DCCH (16 slots for a half-rate DCCH). The logical channels specified in IS-136 include a fast, an extended and a point-to-multipoint short message service broadcast control channels (F-BCCH, E-BCCH and S-BCCH, respectively) for carrying system-related information which is broadcast to all mobile stations, and a short message service, paging and access response channel (SPACH) for carrying information which is sent to specific mobile stations (e.g., paging or text messages).
The standards generally described above (IS-54B and IS-136) are not the only digital standards which have been developed or deployed over the past few years. Other digital standards such as Group Special Mobile (GSM) and Personal Digital Cellular (PDC), which also use TDMA, have been widely adopted in certain parts of the world (e.g., Europe and Japan in the case of GSM and PDC, respectively). Among other differences, the existing standards may use different frequency bands for communications between the base station and the mobile station. For example, IS-54B uses a "single" band which actually consists of a pair of frequency bands, one band for transmission and another band for reception by the mobile station. The transmit band (mobile station to base station) extends from 824 MHz to 849 MHz while the receive band (base station to mobile station) extends from 869 MHz to 894 MHz. Each of the RF channels consists of a transmit center frequency (forward channel) in the range 824-849 MHz and a corresponding receive center frequency (reverse channel) in the range 869-894 MHz. The spacing between the center frequencies of adjacent transmit or receive channels is 30 KHz. Furthermore, the transmit and receive center frequencies for any RF channel are separated by 45 MHz (i.e., each transmit channel is paired with a receive channel that is 45 MHz higher in frequency).
PDC, on the other hand, uses a "dual" band, that is, two bands, each of which (like IS-54B) actually consists of a pair of frequency bands. The first band consists of a receive band (base station to mobile station) extending from 810 MHz to 828 MHz, and a corresponding transmit band (mobile station to base station) extending from 940 MHz to 958 MHz. Each of the RF channels in the first band consists of a receive center frequency in the range 810-828 MHz, and a corresponding transmit center frequency in the range 940-958 MHz, with the two center frequencies being separated by 130 MHz (i.e., each receive channel is paired with a transmit channel that is 130 MHz higher in frequency). The second band consists of a receive band (base station to mobile station) extending from 870 MHz to 885 MHz, and a corresponding transmit band (mobile station to base station) extending from 925 MHz to 940 MHz. Each of the RF channels in the second band consists of a receive center frequency in the range 870-885 MHz, and a corresponding transmit center frequency in the range 925-940 MHz, with the two center frequencies being separated by 55 MHz (i.e., each receive channel is paired with a transmit channel that is 55 MHz higher in frequency). The spacing between the center frequencies of adjacent channels is 25 KHz in both the first and second PDC bands.
A mobile station operating according to the PDC standard must be able to transmit and receive (i.e., "transceive") in both the first and the second bands. FIG. 4 shows a typical design for a transceiver which can operate in the first PDC band (this design also may be used for operation in a single band as specified in AMPS or D-AMPS). An incoming (received) signal in the 810-828 MHz range is passed through a band pass filter (BPF) 30 which attenuates out-of-band signals and noise. The output of the BPF 30 then is mixed with the output of a main channel synthesizer (first local oscillator) 32 in a mixer 34 to produce a pair of sum and difference frequencies, as well known in the art. These signal products are passed through a BPF 36 which filters out the (higher) sum frequency leaving only the difference (lower) frequency. The effect of this first mixing and filtering stage is to downconvert the received signal into a first intermediate frequency (IF) signal, which is presented at the output of the BPF 36. This first IF signal is further downconverted into a second IF signal by mixing it with the output of an auxiliary synthesizer (second local oscillator) 38 in a mixer 40, and then filtering the output of the mixer 40 in a BPF 42 so as to select the lower frequency from the mixer 40.
As also shown in FIG. 4, the main channel synthesizer 32 can be used in conjunction with a transmit offset synthesizer 44 to upconvert a baseband signal into a signal in the desired 940-958 MHz range. The baseband signal, which may be comprised of in-phase (I) and out-of-phase (Q) components, is fed to an IQ modulator 46 which modulates the baseband signal onto a carrier signal provided by the transmit offset synthesizer 44. The output of the IQ modulator 46 is mixed in a mixer 48 with the output of the main channel synthesizer 32, and the output of the mixer 48 is passed through a BPF 50 so as to select the desired frequency from the mixer 48.
The circuit of FIG. 4 can be configured to receive or transmit in any RF channel within the first PDC band by appropriate setting of the main channel synthesizer 32, the auxiliary synthesizer 38 and the transmit offset synthesizer 44. For example, if the desired transmit and receive frequencies are 940 and 810 MHz, respectively, the main channel synthesizer 32 can be set to operate at 1008.85 MHz. The mixer 34 will generate a sum frequency signal at 1818.85 and a difference frequency signal at 198.85 MHz. The higher frequency is filtered out in the BPF 36 and the lower frequency is mixed with the output of the auxiliary synthesizer 38, which is set to operate at 198.4 MHz. The mixer 40 will generate a sum frequency signal at 397.25 MHz and a difference frequency signal at 0.45 MHz (450 KHz). The higher frequency is filtered out in the BPF 42 and the lower frequency is delivered for further RF processing (not shown). In the transmit direction, the transmit offset synthesizer 44 may be set to operate at 68.85 MHz. This carrier frequency is modulated in the IQ modulator 46 and mixed with the 1008.85 MHz signal from the main channel synthesizer 32. The mixer 48 will generate a sum frequency signal at 1077.7 MHz and a difference frequency signal at 940 MHz. The higher frequency is filtered out in the BPF 50 leaving the desired transmit frequency at 940 MHz for delivery to an antenna (not shown).
Extending the circuit of FIG. 4 to operation in the second PDC band presents difficulties because of the different transmit-receive (TX-RX) channel separations (130 MHz in the first PDC band and 55 MHz in the second PDC band). These difficulties are best understood in terms of the following relationships: EQU f.sub.s =f.sub.t -f.sub.r =(f.sub.m -f.sub.o)-(f.sub.m -f.sub.i)=f.sub.i -f.sub.o
where f.sub.s is the TX-RX channel separation (or split), f.sub.t is the TX frequency, f.sub.r is the RX frequency, f.sub.m is the frequency of the main channel synthesizer 32, f.sub.o is the frequency of the TX offset synthesizer 44 and f.sub.i is the first intermediate frequency (IF) at the output of the BPF 36. It will be readily appreciated from the above equation that if the TX-RX channel separation is changed to accommodate the second PDC band, either the first IF or the frequency of the TX offset synthesizer 44 must change in the circuit of FIG. 4. Economic considerations generally dictate that the first IF be a single, fixed frequency. Hence, in the prior art, the circuit of FIG. 4 was modified by adding a second transmit offset synthesizer 52 and a switch 54 to switch between the first and second offset synthesizers 44 and 52, respectively, when switching operation between the first and second PDC bands, respectively, as shown in FIG. 5. Alternatively, the prior art sought to avoid the duplication of components by replacing the two offset synthesizers in FIG. 5 with one frequency agile synthesizer which can "hop" between two different frequencies for operation in the first and second PDC bands, respectively. Both of these prior art approaches, however, increase the cost and complexity of the circuit.
In light of the shortcomings of the prior art, there is a need for a frequency synthesis circuit which is suitable for operation in two different frequency bands having different TX-RX channel separations (such as but not limited to the first and second PDC bands described above), but which has lower cost and reduced complexity as compared with prior art circuits. Such a circuit is provided by the present invention. As will be seen, the approach of the present invention also can be used to reduce circuit cost and complexity in mobile stations which operate in a single frequency band.